It is possible inner parasites to have started living in symbiosis with mammals’ embryos and their gene to have become united. Thus, parasites fetched eventual embryos onto papillae, as meantime they fed them to a definite stage.

In Australia “marsupiality” is epidemic. The dingo dog is an exception, but it is in-brought in the not-so-far past by human settlers. What such an epidemic could have been due to, according to this theory? It is known in Australia sweet-water sources can be difficultly found. When there was drought, many animals drank water from scarce water sources and polluted them with their excrements. Thus they conveyed inner parasites. These parasites had affinity to the specific odor of mammals. We can conditionally call them “galactopahagi” or “lactophagi”.

But how have inner parasites selected mammals with specific slits (whales and dolphins- too, have parasitic fishes in their slits), in order to live and feed there? How have they erased parent species? It is known placental mammals feed through their navel cord. The mentioned parasites got attached to it and they sucked food saps. And when the navel cord got disintegrated, they went on sucking from the sprig’s organism. The navel started becoming a slit, in which papillae got formed. It became as a two-way road for food substances – in embryo stage it accepted, and in mature – gave off. It is not obligatory parent species to be placental (mammals), but it became necessary embryos (in an early stage) to “attack” the parasite, in order they not to be eaten by it. The big populations of animals suppose even a bigger epidemic of “lactophagi”. Parasites ate embryos of the non-adapted. But the blastocysts of some animals entered the parasites themselves and went on living in them as they were even brought to the milk. It is known embryos in marsupials can hold their development. Here we see parity between the parasite and “the secondary parasite” in it – there is a lack of food, but they wait together. If the embryo tried to come out of the symbiosis and to feed and grow, it fell prey to “uncontaminated parasites”. Outside however when it is much drier, and not everywhere there was access to food, competition lost chances.

The possibility of womb contamination with parasites in marsupials is high, because of the cloaca.

Some larvae since a milleniums-long symbiosis may have got the attributes of someone else's chromosomes.It is interesting sex chromosomes in monotremes exist as something like of someone else's genes package.